Abu Dawud Book 2, Hadith Number 1359.

Chapter : On the number of rakahs of the prayer at night.

Narated By N/A : The client of Ibn ‘Abbas said that he asked him: How would the Apostle of Allah (PBUH) pray during the night? He replied: I spent a night with him when he was with Maimunah. He slept and woke up when half the night or one-third of it had passed. He stood up and went to a leather-bag containing water. He performed ablution and I also performed ablution with him. He then stood up and I also stood at his left side. He made me stand at his right side. He then put his hand upon my head, as he was touching my ear and awakening me. He then prayed two light rak’ahs and recited Surah al-Fatihah in each of them, and uttered the salutation. He then prayed eleven rak’ahs observing witr and slept. Then Bilal came to him and said: Prayer, Apostle of Allah. He got up and prayed two rak’ahs, and then led the people in prayer.